Vine: “Natural gas is a technology that can be brought on very quickly so it can be back-up or support for renewable electricity sources.”

Wind currently generates about five percent of all electricity in the U.S. but in some states, it’s much higher. Kansas and South Dakota each get more than 20 percent of their electricity from wind. In Iowa, it’s more than 35 percent.

So as clean technology and renewable energy become more cost-effective, coal faces an increasingly challenging future.